Field of the Invention
Embodiments of the invention generally relate to a medical device that evaluates a temperature signal.
Description of the Related Art
Catheters are often used nowadays in medical applications in order to reach regions or cavities of a patient that are located within the body. Catheters are generally flexible tubes, which may carry out various functions depending on their design. For example, a catheter may be equipped with electrodes in order to transmit energy to tissue arranged around the electrodes. Catheters having electrodes are typically used for example for renal denervation, in which afferent sympathetic nerve fibers (of the sympathetic nervous system) in the renal arteries are selectively severed or cauterized by the transfer of thermal energy to the nerve fibers. Renal denervation is generally used for the therapy of therapy-resistant arterial hypertension, which is defined as high blood pressure, which, in spite of drug-based therapy with antihypertensive drugs, including at least one diuretic, is typically not in the desired target blood pressure range.
The process of checking the exact amount of energy that is transferred from the electrodes to the tissue is generally impeded by the fact that the position of the catheter at the site of use cannot be accurately checked. As a result, the catheter, in certain circumstances, is not in direct contact with the tissue to which the energy is to be transferred, and some of the energy is instead transferred to the blood. This may typically lead to coagulation or clumping of the blood, which has to be absolutely avoided in view of safe therapy. In this case, the clumped blood may cause damage to the organs due to the closure of blood vessels, and in the worst case scenario may also close brain or heart vessels, which may lead to the death of the patient. Wall contact between electrodes and tissue is therefore essential for successful execution of the renal denervation procedure. A strategy for avoiding damage is to increase the energy transfer in steps in order to generate a slower temperature rise, which generally poses a lower risk of causing the blood to clump.
To determine the wall contact, the temperature rise over time or the impedance of the tissue is generally measured, since the impedance typically decreases with longer duration of the energy transfer to the tissue. Animal tests have shown however that, by evaluating the impedance alone, it may not be possible to come to a conclusion regarding the presence of reliable wall contact. The lack of wall contact may be measured for example by a low or absent temperature rise after energy transfer. The evaluation of the temperature rise is generally time-consuming however, which is a disadvantage with use as a switch-off criterion.
European Patent 0566725 entitled “Ablation Electrode With Insulated Temperature Sensing Elements”, to Stuart et al., presents an ablation electrode for ablation catheters having thermally insulated thermal sensors. The ablation catheter of Stuart et al. has an energy-emitting body, connected to an energy source, for contacting tissue, the body containing temperature sensors which are arranged on the body and are thermally insulated therefrom. The temperature sensors of Stuart et al. measure the temperature of the tissue and are connected to a high-frequency generator, which generates the energy for the energy-emitting body. According to Stuart et al., the energy transfer of the high-frequency generator may be adapted in accordance with the measured temperature at the temperature sensors or a measured impedance of the tissue in order to obtain a temperature of the tissue in specific temperature ranges or impedance ranges.
For example, European Patent 2338430 entitled “Catheter With Strain Gauge Sensor”, to Govari, discloses a medical probe having strain gauge sensors. The probe of Govari includes a flexible insertion tube, of which the distal end is designed to be brought into contact with tissue of a body cavity, and a sensor tube, which contains an elastic material in the distal end of the flexible insertion tube, the material being designed to deform in response to forces acting from the tissue onto the distal end. According to Govari, a multiplicity of strain gauge sensors are applied at various points to the surface of the sensor tube and generate a respective signal in response to deformations of the sensor tube. The probe further includes at least one temperature-compensating strain gauge sensor, which is designed to generate signals in order to compensate for changes in temperature in the multiplicity of strain gauge sensors.
European Patent 1827277 entitled “Catheter With Multiple Microfabricated Temperature Sensors”, to Nakagawa et al., presents a catheter having a plurality of microfabricated temperature sensors. The temperature sensors of Nakagawa et al. are arranged in a vicinity of a tip of a distal end of the catheter on an outer surface. The tip is formed by an electrode. According to Nakagawa et al., the temperature sensors contain a thin sensor layer, of which the resistance may change due to a temperature change. Furthermore, a temperature sensor may surround the periphery of the catheter.